Organosilanes are widely used in the electronics industry. For example, tetraethylorthosilicate (TEOS) is used as a major source for depositing silicon dioxide. Group 13 (IIIA) and 15 (VA) elements behave as impurities in obtaining silicon dioxide films. One of the challenges for the industry involves removal of impurities effectively from TEOS to make ultrahigh purity TEOS (see Hawley's Condensed Chemical Dictionary, 11th ed., N. Irving Sax, et al. (1987) for the current numbering in the Periodic Table of the Elements).
J. M. Rosamilia, 1994 Proceedings Institute of Environmental Sciences, pp156 demonstrate some of the methods of analysis and impurity levels in TEOS. They also describe the difficulty in removing boron impurities because it forms volatile triethoxyboron complex which distills along with TEOS.
Ultra high purity precursors are extremely important for the fabrication of high performance devices in the semiconductor industry. As the device density increases with the number of transistors, the dimensions of the device decrease to submicroscopic features. At 0.25 .mu.m technology and beyond, even very low levels of impurities (ppb to parts per trillion) effect the device in high failure rates and low performance.
In addition to the miniaturization and design of multilevels, the thickness of the dielectric layers between the levels is continuously reduced. This reduction in thickness of the dielectric layers is required for devices to operate at high speeds in the gigahertz regime. As a result even small amounts of impurities diffuse readily at high temperatures through these dielectric layers and cause device failures. In the case of boron and phosphorus impurities, this effect is dramatic at high process temperatures. This is because light atoms, such as boron (B.sup.11) impurities, easily diffuse through shallow junctions.
Some of the other problems which are caused by the presence of impurities are:
1) Leakage current at junctions.
2) Unstable electrical characteristics of silicon dioxide.
3) Localized eutectic points with silicon o form undesirable alloys.
Highly sensitive techniques are routinely utilized in analyzing all the materials. Instrumentation such as atomic pressure ionization mass spectroscopy (APIMS) Inductively coupled plasma mass spectrometer (ICPMS) and Inductively coupled plasma atomic emission (ICPAE) are used to detect low levels of impurities in the range of &lt;5 ppb.
Typical reactions that can be expected with boron are described in Advanced Inorganic Chemistry, 5th ed., F. Albert Cotton and Geoffrey Wilkinson, 1988, pp. 162-207.
Removal of boron and other impurities in water was earlier demonstrated by use of a boron specific resin (Amberlite IRA -173 Ion Exchange Resin Separation Technology bulletin, Rohm & Haas (1989). Ion exchange resins which contain N-methylglucamine have been used specifically to bind boron as boric acid. This technique has been used in removing boron from irrigation waters and solutions, see Robert Kunin and Albert F. Preuss, I&EC Product Research Development, Vol 3., No 4., (1964). pp 304.
High purity alkoxysilanes have been purified by using chelating resins such as Chitosan followed by vacuum distillation and diffusion with inert gases. The chelating resin contained groups such as HN(CH.sub.2 COOH).sub.2, see Japanese Patent JP 04082892 A2 920316 (1990). Here, they demonstrate removal of impurities such as Na, K, Ca and Cu.
Typically, TEOS is manufactured using ultra high purity starting materials to minimize contaminants. Chlorine, sodium and potassium are some of the common impurities. Halides were removed in alkoxysilanes by reacting with zinc metal to provide purified alkoxysilanes, see U.S. Pat. No. 5,104,999.
The synthesis and manufacture of TEOS for the semiconductor industry is carefully monitored by using high purity silicon containing starting materials and impurity free reagents. The product thus obtained is further purified by fractional distillation. These fractional distillations result in loss of yield in the form of prefractions (25-30%). Even after several repeated distillations TEOS may still contain contamination above 50 ppb.
The prior art generally involves use of ion exchange resins to reduce Group 13 impurities. Other techniques involve separation of boron impurities from aqueous media and purification of alkoxysilanes involved repeated fractional distillation and use of high purity starting materials.
The present invention overcomes the deficiencies in the prior art purification of organosilanes of Group 13 and 15 impurities by providing a technique for producing high purity products using reagents soluble in the product which are less volatile than the product. This results in decreased distillation losses and higher purities, as described in greater detail below.